The present invention relates generally to a technique for varying signal transmission delay times and increasing signal reach within a substrate and, more particularly, to a technique for using air channels within a multi-layer substrate for varying signal transmission delay times and increasing signal reach.
The present state of the art is shown in FIG. 12 and is a typical multi-layer substrate. The typical substrate includes multiple dielectric layers 1, 2, 3, 4, 5, 6, 7, 8, and 9. Between the dielectric layers are multiple signal tracks 10, 11, 12 and 13. Metal reference layers, including ground layers 20, 21 and 22 are also formed between the dielectric layers. The metal reference layers additionally include a power layer 25 provided between the dielectric layers 5 and 6, and a primary layer 28 and a secondary layer 29, which form the outermost layers of the substrate.
Signal loss over the distance of the tracks of the signal layers 20-22 frequently occurs with high bit rates or high signal frequencies. Furthermore, the signals with high bit rates or high frequencies often become skewed over the track distance.
When signals with high bit rates or high frequencies are used, such as digital signaling at 10 Gb/s or higher, long signal tracks can result in significant delays.
Signal velocity (v) is calculated as follows:
v=c/∈xe2x80x83xe2x80x83(2)
where c is equal to the speed of light and ∈ is the dielectric constant. When a typical dielectric having a dielectric constant ∈=3.9 is used, signal velocity is reduced to approximately half the speed of light.
With regard to signal reach, the wavelength (xcex) of one bit of information is:
xcex=c/(f∈)xe2x80x83xe2x80x83(1)
where c is equal to a speed of 3.0xc3x971010 cm/s, f is equal to a frequency of 10xc3x97109 and ∈ is equal to the dielectric constant of a commonly used substrate material, which is commonly between 3.0 and 4.7. Accordingly, the larger the dielectric constant, the shorter the signal reach.
Suspended substrate striplines have been used to minimize the above-identified problem of losses in striplines. Prior U.S. Pat. Nos. 4,521,755, 4,614,922, and 5,712,607 disclose the use of suspended substrate striplines. In all of the aforementioned patents, which are hereby incorporated by reference herein, the striplines are attached to a substrate which is mounted so as to be surrounded by air on both sides.
In U.S. Pat. No. 4,521,755, the disclosed structure is intended to promote uniform current density and lower losses. In the structure disclosed in U.S. Pat. No. 4,521,755, a channel is formed inside of a conductor. A substrate having striplines is mounted inside the channel.
In U.S. Pat. No. 4,614,922 an upper housing having an upper channel and a lower housing having a lower channel are provided. A center board is positioned between the conductive housings. A transmission strip and a conductive surface are formed on the center board. The structure is intended for use in a microwave circuit.
U.S. Pat. No. 5,712,607 discloses the use of an air-dielectric stripline which includes a dielectric layer sandwiched between two spacer layers. Conductive traces are attached to the dielectric layers. Channels are formed in the spacer layer.
None of the aforementioned patents discloses the use of air channels in multi-layer substrates in order to synchronize signals or more generally, the use of channels for the adjustment of signal transmission times through different signal tracks within the substrate. The previous suspended striplines have employed a mechanical construction, which is too bulky for high density circuit packages.
In view of the foregoing, it would be desirable to provide a technique for synchronizing signals and adjusting signal transmission times within a multi-layer substrate which overcomes the above-described inadequacies and shortcomings. More particularly, it would be desirable to provide a technique for providing air channels for synchronizing signals within a multi-layer substrate in an efficient and cost effective manner.
According to an aspect of the present invention, a multi-layer substrate is provided. The multi-layer substrate comprises a first dielectric layer having a first dielectric constant and a first stripline disposed adjacent the first dielectric layer. The multi-layer substrate further comprises a second dielectric layer having a channel therein, the channel filled with a substance having a second dielectric constant different from the first dielectric constant and a second stripline adjacent the channel of the second dielectric layer.
According to another aspect of the present invention, a technique for providing a substrate in which signals are transmitted at more than one propagation speed is provided. In one embodiment, the technique is realized by a substrate having striplines with differing signal propagation speeds, the substrate comprising multiple dielectric layers. A first stripline is disposed adjacent a first dielectric layer having a first dielectric constant and has a first signal propagation speed. A second stripline is disposed adjacent a second dielectric layer having an air channel therein, the air channel having a second dielectric constant different from the first dielectric constant. The second stripline has a second signal propagation speed different from the first signal propagation speed.
In accordance with an additional aspect of the invention, a method for forming suspended striplines within a multi-layer substrate is provided. The method comprises the steps of forming a first substrate layer having conductive material on one side, etching the conductive material into a set of striplines, and applying a second substrate layer over the conductive material. The method further comprises forming a channel in one of the substrate layers and attaching a third substrate layer to the substrate layer having the channel.
The present invention will now be described in more detail with reference to exemplary embodiments thereof as shown in the appended drawings. While the present invention is described below with reference to preferred embodiments, it should be understood that the present invention is not limited thereto. Those of ordinary skill in the art having access to the teachings herein will recognize additional implementations, modifications, and embodiments, as well as other fields of use, which are within the scope of the present invention as disclosed and claimed herein, and with respect to which the present invention could be of significant utility.